The production of ever-increasing heavy oils of high naphthenic acidity and high viscosity are a challenge for the petroleum industry. High organic acid-content oils are highly corrosive towards equipment used in their processing.
At present, some mixtures of Brazilian oils have shown themselves more corrosive than those usually refined, making that units, chiefly distillation units, require modifications for installing more resistant materials. Since it is not possible to precisely measure the corrosiveness of each petroleum oil, it is not possible to previously ascertain which material should make up the unit in order to withstand a considerable corrosion rate. The chosen material should be as adequate as possible, since excessive corrosion leads to risks linked to leakages, to the premature substitution of equipment and to production interruptions.
On the other hand, chemically resistant materials such as stainless steel are expensive, which is reflected on the overall unit cost as well as on the invested capital return.
The reduction of naphthenic acidity, besides avoiding the problems associated to corrosion, improves the efficiency of the desalting unit/electrostatic treater, one of the most important equipment in production or refining units.
The wrong operation of this equipment or the low efficiency of such treatment causes serious damage to the process units, limiting the campaign period and increasing the processing cost, the main consequences of it being:    a) Equipment corrosion, the more heavily affected in fractioning units being top condensers, top region internals, pipes, control valves, top vessel and the tower wall itself;    b) Salt deposition in exchangers and furnaces, since for many of them solubility is reduced with temperature increase, and/or there is water vaporization with salt precipitation; as a consequence, there is an increase in feed loss in the pre-heating battery, lower thermal exchange efficiency, causing increase in fuel utilization in the furnaces, and also contributing to coke deposition within the furnace tubes and to feed limitation of the unit;    c) Excess water in the process stream, with increased fuel consumption for heating and vaporizing it, besides process instability due to water pockets;    d) Increased chemical product utilization for pH control and corrosion inhibitors, besides making more difficult the control for such products addition due to instability of chloride contents, which makes the additions some times excessive, and some times insufficient;    e) Presence of salts, sediments and solids causing catalyst poisoning and worsening the fuel oil or asphalt quality.
Among the several approaches already adopted for reducing the naphthenic acidity of petroleum oils and their fractions are the use of mixtures of oils of different acidity levels, corrosion inhibitors, thermal treatments and hydrotreatment.
In the case of thermal treatment, Examples cited in U.S. Pat. No. 6,086,751 point out to 90% reductions in TAN (Total Acid Number) for some Venezuelan crudes. The crude is at first submitted to a flash for removing water and thereafter the effluent is submitted to temperatures between 340° C. and 420° C., at pressures lower than 7.5 MPa and reaction times up to 2 hours. Under conditions of conventional visbreaking the reduction reaches 80%. This alternative implies equipment expenditure, such as furnaces and soaker vessels.
A further way of naphthenic acidity reduction is to hydrotreat petroleum under mild conditions. However, the unit campaign period can be limited by catalyst contamination and hydrogen consumption is high at the end, which adds cost to refining.
Another alternative is the esterification of the naphthenic acids through alcohol addition, with or without catalysts. However, this route requires high alcohol expenditure, with increased cost.
The application of corrosion inhibitors is another solution adopted to by-pass the acidity problem. Thus, U.S. Pat. No. 5,182,013 teaches that organic polysulfides are effective inhibitors of corrosion caused by naphthenic acids in refinery distillation units.
U.S. Pat. No. 4,647,366 teaches the addition of oil soluble products of an alkynediol and an polyalkylene polyamine as naphthenic corrosion inhibitors.
Acidity reduction can still be obtained through the treatment of oil with basic NaOH or KOH solutions as taught in U.S. Pat. No. 4,199,440. However this approach demands the use of rather concentrated basic solutions and a critical point is the formation of emulsions difficult to separate. Therefore this solution would be applicable only for low base concentrations.
To by-pass the emulsion problem, U.S. Pat. No. 6,054,042 teaches a treatment with an overbased detergent containing at least 3% calcium wherein the overbased detergent is selected from the group consisting of calcium sulfonates and phenates. Oil is treated at temperatures between 100° C. up to 170° C., the amount of overbased detergent being from 0.025:1 to 10:1 moles of calcium to acidic functionality in the starting crude oil. Amounts of 0.25:1 can also be used.
U.S. Pat. No. 6,258,258 teaches the use of anhydrous ammonia solutions. The proposed treatment can be carried out in two steps, with a first step under conditions of temperature and period of time sufficient to form the ammonium salts of the naphthenic acids and with a second step where the ammonium salts are treated under conditions of temperature and period of time adequate to form the naphthenic acid amides.
U.S. Pat. No. 6,281,328 teaches the use of polymeric amines such as polyvinyl pyridine to solve the problem of naphthenic acidity.
U.S. Pat. No. 4,300,995 teaches the treatment of coal and coal-derived liquids besides vacuum gasoils and petroleum residua showing acidic functionalities with basic solutions of quaternary hydroxides in alcohol or water, such as tetramethyl ammonium hydroxide in a liquid such as alcohol or water.
International publication WO 01/79386 teaches a basic solution containing Groups IA, IIA and ammonium hydroxides and the application of a transfer agent, such as non basic quaternary salts and polyethers to solve the problem of naphthenic acid reduction.
In U.S. Pat. No. 6,190,541 the hydroxide and/or phosphate bases are used with an alcohol for the desired reduction in the naphthenic acid content.
In U.S. Pat. No. 5,985,137, naphthenic acidity and the sulfur content of the oil are reduced by reaction with alkaline earth metal oxides yielding neutralized compounds and alkaline earth metal sulfides. The temperature should be higher than 150° C. for the removal of the carboxylic acids and higher than 200° C. for forming sulfide salts. The applied pressure should keep the material in a non-vaporized state.
Broadly, most of the methodologies used for reducing naphthenic acidity involving thermal treatments without or with the addition of basic solutions, demand the application of surfactants to by-pass the emulsion problem.
A still different approach is the use of adsorbents for adsorbing the naphthenic acids.
Thus Brazilian application PI 0202552-3 of the Applicant teaches the reduction of naphthenic acidity of petroleum oils or their fractions that have been previously submitted to desalting and dehydration through a process that comprises the steps of: a) contacting the naphthenic acid—containing oils or their fractions with an adsorbent, at a ratio of adsorbent/petroleum oil or its fractions in the range of 0.1 to 5, at temperatures between 200° C. and 500° C., under pressures between 0.01 to 0.3 MPag and residence time between 1 second and 2 hours, in order to carry out the desired reduction in naphthenic acidity and obtaining a treated feed; b) on the so-obtained treated feed, separating the spent adsorbent from the petroleum oil or its fractions so as to obtain a treated and separated feed of reduced naphthenic acidity; and c) directing the treated and separated feed for further treatment. The adsorbent used in said Brazilian application is a high specific area material, between 100 and 200 m2/g, the surface of said material being covered with a layer of high molecular weight carbon compounds. Useful adsorbent compounds are carbon black, FCC spent catalysts and coked FCC catalyst.
U.S. Pat. Nos. 4,582,629 and 4,853,119 propose the use of microwaves for emulsion breaking. However, there is no description nor suggestion in these patents as for the removal or reduction of naphthenic acidity.
U.S. Pat. No. 6,454,936B1 teaches the reduction of the amount of naphthenic acids contained in oils by forming an oil/water (O/W) emulsion and using solids. The oil is at first treated with between 0.1 to 5 wt % based on the weight of oil, of a solid able to adsorb the acids present in the oil. Useful solids are silica, alumina, coke, montmorillonite, bentonite, kaolinite and the like. The solids should be of an amphiphilic nature, that is, show a hydrophilic/lipophilic character. The solids are added of 5 to 30 wt % of water based on the amount of oil, at temperatures between 20 to 220° C., the preferred range being 25° C. to 80° C., for 3 to 30 minutes, under pressure between 413.7 kPa up to 6,895 kPa. Water is then added to form an emulsion and separated in a plurality of layers. Separation can be effected through any well-known process such as centrifugation, gravity decantation, hydrocyclones, microwaves, electrostatic separation and combinations of these methods.
In spite of the fact that U.S. Pat. No. 6,454,936B1 mentions the use of microwaves for separating the emulsion, the object of the technology taught therein is not the use of microwave for reducing the naphthenic acid content of the oil, it being restricted to the emulsion separation. It is the solid added to oil before forming the emulsion that is designed to adsorb the naphthenic acids, as pointed out in column 3, line 2 of the cited patent. Since naphthenic acids are of amphiphilic character, the amphiphilic solid will easily adsorb the naphthenic acids.
In spite of the good results related to the naphthenic acid reduction using solid adsorbents reported in the literature, a drawback of this technology is the introduction of an additional separation step for separating the solid adsorbent. Besides, the mere fact of adding to the feed to be treated a foreign material—the adsorbent solid—means cost and trouble to the system. A further drawback is that adsorbents are most of the time used under severe temperature and pressure conditions, these entailing increased cost.
Another relevant and unexpected aspect of the invention, not described nor suggested in the literature is that state-of-the-art processes must submit the overall feed to the proposed treatment, for example, to heating, for a certain residence time, which in general terms is of one hour. Advantageously, in the invention the required energy is used to heat only a fraction of the feed (that is, the droplet emulsified or dispersed in the hydrocarbon phase), for a residence time lower than that required in known processes.
Without being linked to any particular theory, the Applicant hypothesizes that a further advantage of the invention lies in the fact that the naphthenic compounds where the chain shows a reduced number of carbon atoms, being relatively more polar and of higher acid strength than longer or more complex chain analogous compounds, will have more affinity and will concentrate at the interface of the aqueous phase, this allowing higher effectiveness of the intended reduction.
Thus, it can be seen that in spite of the technical developments in this field the technique still needs a process for reducing the naphthenic acid content of petroleum oils and their fractions, said process involving the treatment of said feeds, in the desalting or dehydration steps, with electromagnetic energy in the microwave range, in liquid phase, at temperatures between 50° C. and 350° C., with separation of any formed gas phase, such process being described and claimed in the present application.